Float glass surface modification process



Sept. 16, 1969 0. G. LOUKES ET AL 3,467,508

FLOAT GLASS SURFACE MODICATION PROCESS Filed July 6, 1966 3 Sheets-Sheetl z -N- M 3 3 3 If @M N2 w /M v k g w M 4 CAL C M. & W V v 6M 5 m w|l||| w. "m MW .W..@\L\ vme w Q M t 9 mm mm. m w

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I'll! $9M mm B m? Q mm Qow m P u u w RN N 7 RN RN mm -Wlmi I lnvenlo rsv I v v mtlomeys D. G. LOUKES ET A FLOAT GLASS SURFACE MODICATIONPROCESS 5 Sheets-Sheet 2 Sept. 16, 1969 fiymdm, flmz gmmms Sept. 16,1969 e. LOUKES ETAL 3,467,508

FLOAT GLASS SURFACE MODICATION PROCESS gum g United States Patent US.Cl. 65-30 17 Claims ABSTRACT OF THE DISCLOSURE Glass having desiredcharacteristics is manufactured by separately contacting the surfaces ofthe glass with segregated bodies of molten electrically conductivematerial and passing a controlled electric current through the glassbetween the bodies to cause ionic migration into the glass.

This invention relates to the manufacture of flat glass and moreespecially to the manufacture of flat glass during which glass is incontact with molten metal, for example during which glass is in contactwith molten metal, for example during which glass is supported on amolten metal surface such as the surface of a bath of molten metal.

It is a main object of the present invention to provide an improvementin the manufacture of flat glass having desired surface characteristics.

According to the invention a method of manufacturing flat glass havingdesired surface characteristics comprises separately contacting thesurfaces of the glass with bodies of molten electrically conductivematerial, and employing these bodies of molten material as electrodes topass a controlled electric current through the glass and thereby modifythe surface constitution of the glass.

In a preferred method according to the invention one surface of theglass is contacted with a molten metal, molten electrically conductivematerial is confined against the other surface of the glass, and thesurface constitution of the glass is modified by passing a controlledelectric current through the glass between the confined molten materialand the molten metal.

Desirably the glass is supported on a molten metal surface, and themolten electrically conductive material is confined on the upper surfaceof the glass.

The molten material confined on the upper surface of the glass may be amolten salt or a molten metal and must be isolated from the molten metalon which the glass is supported in order that the only electrical pathshall be through the thickness of the glass. The electric current may bea direct current so that there is preferential treatment of one of thesurfaces of the glass to modify its characteristics. Alternatively,alternating current may be employed to change simultaneously the surfacecharacteristics of both surfaces of the glass.

The invention may be employed for the treatment of individual sheets ofglass, or for modifying the surface characteristics of a continuousribbon of glass. Molten metal may be employed to provide the necessaryelectrical contact with the top surface of the fiat glass in ribbon formwhich is advanced along a molten metal surface, a pool of molten metalbeing confined on the upper surface of the glass and thereby inhibitedfrom forward movement with the glass.

In order to isolate the pool of molten metal from the molten metalsurface along which the ribbon of Patented Sept. 16, 1969 ice glass isadvanced the pool must be confined laterally on top of the ribbon ofglass and in addition any tendency of the pool to be carried along onthe ribbon of glass is inhibited so that the pool remains stationaryrelative to the molten metal surface. From this aspect a methodaccording to the invention comprises delivering molten glass at acontrolled rate to a bath of molten metal to establish a layer of moltenglass on the bath, maintaining the layer in molten condition as it isadvanced along the bath, shaping the margins of the layer as it isadvanced so that the ribbon assumes a shallow trough-like configuration,and confining said pool of molten metal on top of the advancing glass inthe shallow trough so formed.

In order to treat the top surface of the glass electric current ispassed through the glass in a direction such that the pool of moltenmetal acts as an anode, and the strength of the current is so controlledthat a desired change in the characteristics of the top surface of theglass takes place.

It has been found that if a suitable concentration of metal can be madeto enter the top surface of the glass then this surface has good lightand heat reflecting properties. To this end the current is so controlledthat a predetermined concentration of metal enters the top surface ofthe glass from the pool.

Modification of the undersuiface of the glass may be desired and fromthis aspect the invention comprehends a method wherein the controlledelectric current is passed through the glass in a direction such thatthe molten metal of the bath acts as an anode and migrates into theundersurface of the ribbon of glass.

Desirably the bath is a bath of molten tin or of a molten tin alloyhaving a specific gravity greater than that of the glass.

As an alternative to tin or tin alloys the molten metal of the bath maybe lead or bismuth or an alloy of lead or bismuth having a specificgravity greater than the glass.

The pool of molten metal confined on the surface of the glass may be apool of molten tin, or lead or bismuth.

Alternatively a pool of a molten alloy of tin, or lead or bismuth may beconfined on the upper surface of the glass. For example the pool may bean alloy of tin with an element selected from the group of elementsconsisting of lithium, sodium, potassium, zinc, magnesium, aluminium,silicon, titanium, manganese, chromium and iron. The alloy may be analloy of tin with one of the rare earth metals. When employing a tinalloy, the relative concentration of the tin and the other metal andtheir relative chemical properties determines whether only the metalalloyed with the tin enters the surface of the glass.

In other applications of the invention the alloy employed may be analloy of bismuth or lead with an element selected from the groupconsisting of lithium, sodium, zinc, magnesium, aluminium, silicon,titanium, manganese, chromium, iron, cobalt, nickel, copper, silver,gold, antimony, arsenic and indium. Further the alloy may be an alloy ofbismuth or lead with an element selected from the group consisting ofthe platinum group metals and the rare earth metals.

Treatment of both surfaces of the glass may be effected according to theinvention by employing a method in which two pools of molten metal areconfined on the upper surface of the glass and are separated by adistance such that the pools are electrically isolated from each other,electric current is passed through the glass between the molten metalsurface and the pools of molten metal in a direction such that one poolacts as an anode and the other pool acts as a cathode with respect tothe molten metal surface, and the strength of both currents is soconrolled that a predetermined concentration of metal enters bothsurfaces of the glass.

The two separate pools may he of the same metal as the bath along whichthe ribbon is advanced or of a different molten metal or alloy. Furtherthe two pools may be different from each other. In addition if it isdesired to apply two successive treatments to the upper surface of theglass the electrical connection to the pools may be such that both poolsact as anodes, or both as cathodes with respect to the molten metal onwhich the glass is supported.

The invention also comprehends apparatus for use in the manufacture offlat glass having desired surface characteristics comprising a tankstructure containing a bath of molten metal, means for delivering glassat a controlled rate to the bath and for advancing the glass along thebath as a layer of molten glass in ribbon form, thermal regulators forcontrolling the viscosity of the glass so as to ensure that the glass isgradually cooled as it is advanced until it is sufficiently stiffened tobe taken unharmed from the bath, means for maintaining a pool of moltenmaterial on top of the glass and for preventing forward movement of thepool with the glass, electrodes arranged to dip into the pool of moltenmaterial and into the bath of molten metal, and an electric currentsupply circuit connected to said electrodes.

In a preferred embodiment the apparatus includes thermal regulators forcontrolling the viscosity of the glass so as to ensure that the layer ofglass is advanced in a plastic state, edge shaping means mountedrelative to the tank side walls for shaping the margins of the ribbon ofglass so that the ribbon assumes a shallow troughlike configuration, andmeans for maintaining the pool of molten material in said trough.

The edge shaping means may be comprised by a pair of edge forming toolsof non-wettable material mounted opposite each other at the sides of thetank, the tools having shaped forming surfaces for engagement by theedges of the ribbon of glass, which surfaces fold the edges of theribbon up and over to form ridges along the margins of the ribbon ofglass.

In order that the invention may be more clearly understood someembodiments thereof will now be described, by way of example, withreference to the accompanying drawings, in which:

FIGURE 1 is a sectional elevation of apparatus according to theinvention including a tank structure containing a bath of molten metal,a roof structure over the tank structure and apparatus for pouringmolten glass on to the bath,

FIGURE 2 is a plan view of the apparatus of FIGURE 1 but with the roofstructure removed,

FIGURE 3 is a plan view of an edge forming tool for shaping the marginsof the ribbon, two such tools being employed in the apparatus of FIGURES1 and 2,

FIGURE 4 is a section on line IVIV of FIGURE 3,

FIGURE 5 is a section through one margin of the ribbon of glass showingthe first stage of the shaping of that margin by the part of the toolillustrated in FIG- URE 4,

FIGURE 6 is a section on line VI-VI of FIGURE 3,

FIGURE 7 is a section through the margin of the ribbon showing the shapeproduced by the part of the tool illustrated in FIGURE 6,

FIGURE 8 is a section through the final form of one margin of the ribbonof glass,

FIGURE 9 is a sectional elevation through a part of the tank structureshowing the application of the invention to a method in which twoseparated pools of molten metal are confined on the top surface of theribbon of glass, and

FIGURE 10 is a plan view of the apparatus of FIG- URE 9.

In the drawings like references indicate the same or similar parts.

Referring to FIGURES 1 and 2 of the drawings, a forehearth of acontinuous glass melting furnace is indicated at 1 and a regulatingtweel at 2. The forehearth ends in a spout 3 comprising a lip 4 and sidejambs 5, one of which is shown in FIGURE 1. The lip 4 and side jambs 5together constitute a spout of generally rectangular cross-section.

The spout 3 is disposed above the floor 6 of an elongated tank structureincluding side walls 7 joined together to form an integral structurewith the floor 6, an end wall 8 at the inlet end of the tank, and an endwall 9 at the outlet end of the tank. The tank structure holds a bath ofmolten metal 10 whose surface level is indicated at 11. The bath is, forexample, a bath of molten tin or of an alloy of tin in. which tinpredominates and the bath has a specific gravity greater than that ofglass.

A roof structure is supported over the tank structure and the roofstructure includes a roof 12, side walls 13 and integral end walls 14and 15 respectively at the inlet and outlet ends of the bath. The inletend wall 14 extends downwardly close to the surface 11 of the moltenmetal to define with that surface an inlet 16 which is restricted inheight and through which molten glass is advanced as will be describedbelow. The outlet end wall 15 of the roof structure defines with theoutlet end wall 9 of the tank structure an outlet 17 through which theultimate ribbon of glass produced on the bath is discharged on to drivenconveyor rollers 18 mounted outside the outlet end of the tank anddisposed somewhat above the level of the top of the end walls 9 of thetank structure so that the ribbon is lifted clear of the wall 9 fordischarge through the outlet 17.

The rollers 18 convey the ribbon of glass to an annealing lehr in wellknown manner and also apply tractive effort to the ribbon of glass toassist in advancing the ribbon as it glides along the surface of thebath 10.

An extension 19 of the roof 12 extends up to the tweel 2 and forms achamber with side walls 20 in which chamher the spout 3 is disposed.

Molten soda/lime/silica glass 21 is poured on to the bath 10 of moltenmetal from the spout 3 and the tweel 2 regulates the rate of How of themolten glass 21 over the spout lip 4. The spout is vertically spacedfrom the surface 11 of the bath so that the molten glass 21 has a freefall of a few inches, exaggerated in FIGURE 1, to the bath surface. Thisfree fall is such as to ensure the formation of a heel 22 of moltenglass behind the glass 21 pouring over the spout, which heel 22 extendsup to the inlet end wall 8 of the tank structure.

The temperature of the glass as it is advanced along the bath isregulated from the inlet end down to the discharge end by thermalregulators 23 immersed in the bath 10, and thermal regulators 24 mountedin the headspace 25 which is defined over the bath by the roofstructure. A protective gas is supplied to the headspace through ducts26 which are provided at intervals in the roof 12. The ducts 26 areconnected by branches 27 to a header 28 which is connected to a supplyof protective gas. Thus a plenum of protective gas is maintained in theheadspace 25 which is a substantially closed headspace and there isoutward flow of protective gas through the inlet 16 and the outlet 17.

The' temperature of the molten glass is regulated by i the thermalregulators 23 and 24 as the glass is advanced along the bath so as toensure that a layer of molten glass 29 is established on the bath. Thislayer 29 is advanced along the bath through the inlet 16 and as it isadvanced there is free lateral flow of the molten glass under theinfluence of surface tension and gravity until there is developed fromthe layer 29 a buoyant body 30 of molten glass, which is then advancedin ribbon form along the bath.

The width of the tank structure at the surface level II of the bath isgreater than the width of the buoyant body 30 of molten glass so thatthere is no limitation to the initial free lateral flow of the moltenglass.

The margins of the ribbon are shaped as it is further advanced to formthe ribbon into a shallow trough or channel in order that moltenelectrically conductive material may be contained laterally on the uppersurface of the ribbon of glass and completely isolated from the moltenmetal bath. One way of shaping the margins of the ribbon of glass toproduce the shallow channel is illustrated diagrammatically in FIGURES 1and 2 and in greater detail with reference to FIGURES 3 to 8. Two edgeforming tools 31 and 32 of a material which is not wetted by moltenglass, for example carbon in the form of graphite, are mounted atopposite sides of the tank structure.

The tools may be water-cooled if desired and are partially immersed inthe molten metal of the bath, as shown in FIGURE 1.

The tools have specially shaped forming surfaces for engagement by themargins of the ribbon of glass. The tools are a mirror image of eachother and a plan view of the tools 31 is shown in FIGURE 3. The shape ofthe forming surface of the tool gradually changes from an upward slope33 as illustrated in FIGURE 4 to an inwardly directed curve as shown at34 in FIGURE 6.

As the buoyant body of molten glass 30 is advanced in ribbon form themargins of the ribbon first ride on to the upwardly sloping parts 33 ofthe tools 31 and 32 and are bent to the formation shown in FIGURE 5which illustrates one margin 35 of the ribbon of glass having an upwardslope matching the initial slope 33 of the tool. Thereafter the slope 33of each tool gradually curls over to assume the shape shown at 34 sothat the margins 35 of the ribbon are both gradually folded up and overas the ribbon is advanced, and by the time the margins of the ribbonpass beyond the downstream ends of the tools 31 and 32 they havecollapsed to the folded formation shown at 36 in FIGURE 8. This foldedformation therefore produces marginal ridges along both edges of theribbon which give to the ribbon a shallow troughlike configuration whichenables molten material to be confined on the top surface of the ribbonof glass 37 as it is advanced along the bath of molten metal. The heightof the marginal ridge 36 formed along one edge of the ribbon of glass 37is exaggerated in FIGURE 1 for the sake of clarity.

The edge forming tools 31 and 32 are preferably cooled and a compositepipe 38 for supplying cooling water to the tool 31 and for exhaustingthe water is indicated in FIGURE 3. The margins of the glass lose heatto the forming tools so that the marginal ridges'36 which are formed aresufficiently stiffened to hold their form when the ribbon of glass 37 isadvanced downstream of the tools.

In an alternative embodiment of the invention glass may be delivered tothe bath at a controlled rate as a formed ribbon of glass which isadvanced on to the bath from casting rollers associated with the inlet16 to the bath. The casting rollers both deliver glass in ribbon form tothe bath and advance the ribbon along the bath towards the outlet endthereof. The casting process for delivering a formed ribbon of glass tothe bath may be elaborated by forming marginal ridges on the ribbon ofglass before it is delivered to the bath.

The present invention is concerned with surface treatment of the ribbonof glass as it is advanced along the surface of the bath of molten metalin order to impart desired surface characteristics to the glass. Nearthe outlet end of the bath where the temperature of the glass is in theregion of 750 to 600 C. a body of molten electrically conductivematerial 39, e.g., a pool of molten metal, is confined on the topsurface of the glass. The sides of the pool 39 are contained laterallyby the marginal ridges 36 and forward movement of the downstream edge 40of the pool with the glass is inhibited by the upward slope of theribbon of glass as it is lifted from the bath surface for dischargethrough the outlet 17.

In the embodiment of FIGURES l and 2 the quantity of molten material inthe pool 39 is such that the pool finds its own equilibrium thickness onthe top surface of the glass, which in the case of molten tin or amolten tin alloy in which tin predominates is about inch, and theupstream edge 41 of the pool remains stationary relative to the tankstructure as the ribbon of glass is advanced underneath the pool.

The material 39 may be a pool of molten tin or a pool of a molten tinalloy for example an alloy of tin with lithium, sodium, potassium, zinc,magnesium, aluminium, silicon, titanium, manganese, chromium or iron.Alternatively the alloy may be an alloy of tin with one of the rareearth metals.

The upstream edge 41 of the pool 39 may be held, for example by a carbonbarrier extending across the ribbon surface just above the surface butwithout touching the surface. This barrier prevents the molten material39 from flowing along the ribbon surface contrary to the direction ofmovement of the ribbon, and the gap between the bottom of the carbonbarrier and the surface of the glass is so small that surface tensionprevents the molten material from escaping through the gap.

Other ways may be employed for confining the pool of molten material onthe ribbon surface, for example the edges of the ribbon may be raised oncarbon skids which are held in the bath surface at the sides of the bathso that the production of marginal ridges 36 would not be necessary.

An electrode 42 carried on a conducting support rod 43 dips into thesurface of the pool 39 of molten material. The electrode 43 may be acarbon electrode or an osmium tipped copper electrode. The rod 43 passesthrough the side wall 7 of the tank structure and has a terminal 44fixed to it. A second electrode 45 is similarly mounted on a connectingrod 46 which is shorter than the rod 43 and ends in a terminal 47. Theelectrode 45 dips into the molten metal bath alongside that part of theribbon on which the pool 39 is confined. The support rods 43 and 46 areinsulated from the tank structure.

The lateral containing of the sides of the pool 39 obviates anypossibility of short-circuiting between the confined pool and the bathof molten metal and through the electrodes 42 and 45 electrical contactis made with that part of the upper surface of the ribbon of glass whichis supporting the pool 39 of molten material, and with the whole of thebottom surface of the ribbon of glass supported on the bath of moltenmetal. The terminals 44 and 47 are connected to a direct current supplyin such manner that the pool of molten tin 39 on the glass acts as ananode and the molten metal bath 10 acts as a cathode.

A direct current of, for example, 50 amps. at a voltage.

of the order of 50 volts is provided by the supply circuit and thiscurrent passing through the thickness of the glass causes tin to migrateelectrolytically from the pool 39 into the top surface of the ribbon ofglass. The current which is passed through the ribbon in order toproduce a desired change in the surface characteristics of the topsurface depends on the speed, in square inches per second, at which theribbon of glass is being advanced. The amount of tin which enters agiven square inch of the topsurface of the glass depends also on thetemperature and thickness of the glass and the applied voltage.

In one method of operating, the length of the pool 39 is such that theribbon of glass takes about 60 seconds to pass under the confined poolof tin, the resulting surface concentration of tin in the top surface ofthe ribbon which is achieved being such that the ribbon taken from thebath has something of a metallic appearance.

The treated glass which is thus produced has unusual heat reflecting andlight transmitting properties and is not easily wetted by water, as wellas having special chemical properties. After the ribbon of glass hasbeen annealed, the margins are trimmed off leaving a ribbon of flatglass having the desired surface characteristics.

The amount of tin migrating into the surface of the glass may be in theregion of 1 milligram of tin per square inch of the glass surface, andwith such a concentration of'tin in the surface the glass noticeablytransmits less light. If a yet higher current is passed through theglass there is a resulting surface concentration of tin in the glass inthe region of several milligrams of tin per square inch and aniridescent grey surface is produced.

The arrangement illustrated in FIGURES 1 and 2 gives a relativelyshallow surface treatment. If a deeper surface treatment of the glass isdesired then the method of the invention may be applied higher up thebath, where the glass is hotter but is not so hot that its upper surfaceis affected "by the weight of the pool of molten material acting on it.

Metal from the bath may be caused to enter the bottom surface of theribbon of glass by reversing the electrical connections to the terminals44 and 47. The molten metal bath then acts as an anode and the pool 39of molten metal acts as a cathode, and the bath metal, e.g., tin, leador bismuth is carried electrolytically into the bottom surface of theglass ribbon. The migration of metal into the glass takes place mainlyinto that part of the bottom of the ribbon which is directly underneaththe pool 39 of molten metal.

The use of tin alloys for the pool 39 has already been mentioned, and byusing different alloys other surface finishes may be obtained for theribbon of glass.

Alloys of bismuth or lead may be used for the pool 39, for examplealloys of bismuth or lead with any one of the following elements, namelylithium, sodium, zinc, magnesium, aluminium, silicon, titanium,manganese, chromium, iron, cobalt, nickel, copper, silver, gold,antimony, arsenic and indium; or with a metal of the platinum group,that is platinum, palladium, ruthenium, rhodium osmium, or iridium; orwith a rare earth metal.

In particular alloys based on bismuth or lead can be employed forcausing a colouring element to enter the top surface of the glass. Forexample the top surface of the glass may be stained yellow by employinga pool of a silver/bismuth alloy, or may be stained grey by a pool ofnickel/bismuth alloy. A red colour is obtained by employing a pool of acopper/ bismuth or a copper/lead alloy, especially if the top surface ofthe glass has already been reduced by hydrogen present in the headspaceover the bath, or if a reducing agent has previously been introducedinto the top surface of the glass in the manner described herein withreference to FIGURES 9 and 10. A pool of a bismuth/copper/tin alloy incorrect proportions may be employed to introduce both copper and tinsimultaneously into the glass surface whereupon a red colour isproduced.

Elements other than the molten metal of the bath may be caused tomigrate into the bottom surface of the ribbon of glass by maintaining inthe molten bath a requisite concentration of the desired element so thatwith the bath as the anode the electrolytic action causes the element tomove from the bath in a controlled manner into the bottom surface of theribbon of glass.

A pool of electrically conductive molten salt may be confined on the topsurface of the ribbon of glass. For example a high surface concentrationof silver may be produced in the ribbon of glass by confining a layer ofa molten silver halide, for example silver chloride, on the top of theglass near the outlet end of the bath of molten metal as illustrated inFIGURES 1 and 2, and then passing a controlled electric current throughthe glass with the pool of silver chloride acting as the anode. Copperor zinc may be caused to migrate electrolytically into the glass from apool of a molten copper salt or a molten zinc salt confined on the uppersurface of the ribbon of glass.

In the embodiments described above direct current is employed but it hasbeen found that an alternating current may be used, for example acurrent alternating at about 1 cycle per second, to cause tin to enterpreferentially into one surface of the glass.

A heating alternating current at mains frequency may be superimposed onthe controlled direct electric current in order to cause electricalheating of the glass thereby enhancing the controlled migration of anelement into the glass surface.

FIGURES 9 and 10 illustrate a form of apparatus according to theinvention for modifying both surfaces of the ribbon of glass. Two moltenpools 48 and 49 are separately confined on the top of the ribbon ofglass being separated by a sufficient distance to ensure that the poolsare electrically isolated from each other. The marginal ridges 36 formedon the ribbon of glass contain the side edges of the pools laterally andthe upstream and downstream edges of the pools are held by means ofcarbon barriers. There are two such barriers 50 and 51 holding the frontand rear edges of the pool 48, and two similar barriers 52 and 53holding the front and rear edges of the pool 49. The carbon barriers 50to 53 are individually and adjustably supported by means of struts 55between the side walls of the tank structure. The mounting of thebarriers 50 to 53 permits both vertical and longitudinal adjustment ofthe barriers relative to the tank structure.

Associated with the pool 48 there are two electrodes 58 and 59 whichrespectively dip into the top surface of the pool 48 and into the moltenmetal bath 10 alongside the pool 48. The electrode 58 is mounted on aninsulated connecting rod 60 fixed in the tank side wall 7 and carrying aterminal 61. The electrode 59 is carried on an insulated connecting rod62 which has a terminal 63. The terminal 63 is connected to one end ofthe secondary winding of a transformer 64 and the terminal 61 isconnected to the other end of that secondary winding through a rectifier65. The primary winding of the transformer 64 is connected to a souce 66of alternating current.

Similarly the pool 49 has asociated electrodes 67 and 68 respectivelydipping into the pool 49 and into the molten metal bath '10 alongsidethe pool. The electrodes 67 and 68 are carried on insulated connctingrods 69 and 70 having terminals 71 and 72. As shown in FIGURE 9 theterminal 71 is connected to one end of the secondary winding of atransformer 73 and the terminal 72 is connected through a rectifier 74to the other end of that secondary winding. The primary winding of thetransformer 73 is connected to a power supply source 75. By the use ofthis method of electrical connection the pool 48 acts as an anode withrespect to the molten metal bath 10, and the pool 49 acts as a cathodewith respect to the bath 10. Thus metal will enter the top surface ofthe ribbon of glass from the pool 48 and will enter the bottom surfaceof the ribbon of glass from the bath 10 as the ribbon passes under thepool 49 so that the ultimate ribbon of glass produced has apredetermined concentration of metal in both surfaces. This method couldbe employed in a process in which the pool 48 is a pool of a lithium/tin alloy, the pool 49 is a pool of tin, and a predeterminedconcentration of lithium is alloyed with the tin of the bath 10. Lithiumenters both surfaces of the ribbon and the glass can be strengthened bysubsequent ion exchange reactions involving the lithium ions.

In another application of the twin-pool embodiment of the invention zincis introduced into the surfaces of the glass in controlled amounts froma tin/zinc alloy in order to improve the weathering properties of theglass.

Both the pools 48 and 49 may have the same polarity with respect to thebath 10 in another application of the invention to a two stage treatmentof the top surface of the glass. For example both the pools may beconnected as anodes, the first pool being of tin or an arsenic/bismuthalloy, and the second pool being of a copper/bismuth alloy or acopper/lead alloy. A reducing agent is introduced into the top surfaceof the glass from the first pool, and the copper entering the glass fromthe second pool is reduced to give a red colour to the glass.

The metal alloyed with the tin, lead or bismuth may be continuallyreplenished by electrolysis from a pool of the salt of that metal whichfloats on top of the pool of molten metal. For example the lithium in alithium/tin alloy may be continually replenished from a pool of alithium salt which is floated on the pool of alloy.

The effect of the surface treatment of the glass may be graduated acrossthe ribbon of glass by shaping the top pool or pools of molten metal toa non-uniform width or length. For example a laterally grade top surfacetreatment of the ribbon is achieved by holding the leading edge of thepool 39 of FIGURES l and 2 at an angle tothe direction of advance of theribbon of glass by means of .a carbon barrier.

Additionally longitudinally graded top or bottom surface treatrnent ofthe glass may be produced by continuously varying the applied voltageand hence the current flowing through the glass.

The invention thus provides an improved method of manufacturing flatglass in which the constitution of either or both surfaces of the bathcan be accurately controlled to produce a desired surface quality of theglass. The invention also comprehends fiat glass produced by the methodof the invention and having a predetermined surface constitution.

We claim:

1. A method of manufacturing glass having desired surfacecharacteristics, comprising separately contacting the surfaces of theglass with bodies of molten electrically conductive material, isolatingthe molten bodies from each other, and passing an electric currentthrough the glass between said isolated bodies to modify the surfaceconstitution of the glass.

2. A method of manufacturing fiat glass having desired surfacecharacteristics, comprising contacting one surface of the glass withmolten metal, confining a segregated body of molten electricallyconductive material against the other surface of the glass, andmodifying the surface constitution of the glass by passing an electriccurrent through the glass between the confined molten material and themolten metal.

3. A method according to claim 2, comprising supporting the glass on amolten metal surface, and confining said segregated body on the uppersurface of the glass.

4. A method according to claim 3, wherein flat glass in ribbon form isadvanced along a molten metal surface, and the molten electricallyconductive material is a pool of molten metal which is isolated on theupper surface of the glass and thereby inhibited from forward movementwith the glass.

5. A method according to claim 4, comprising delivering molten glass toa bath of molten metal to establish a layer of molten glass on the bath,maintaining the layer in molten condition as it is advanced along thebath, and shaping the margins of the layer as it is advanced so that theribbon assumes a shallow trough-like configuration, to laterally confinesaid pool of molten metal on top of the advancing glass in the shallowtrough so formed.

6. A method according to claim 5, wherein the margins of the ribbon areformed as ridges for laterally containing the molten pool.

7. A method according to claim 4, wherein electric current is passedthrough the glass, in a direction such that the pool of molten metalacts as an anode, to effectuate a desired change in the characteristicsof the top surface of the glass.

8. A method according to claim 4, wherein flat glass is advanced inribbon form along a bath of molten metal, selected from the groupconsisting of molten tin and molten tin alloys, and. the electriccurrent is passed through the glass in a direction such that the moltenmetal of the bath acts as an anode and migrates into the undersurface ofthe ribbon of glass.

9. A method according to claim 4, wherein the pool confined on the uppersurface of the glass is a pool of molten metal selected from the groupconsisting of tin, lead and bismuth.

10. A method of manufacturing flat glass having desired surfacecharacteristics, comprising advancing flat glass in ribbon form along amolten metal surface, isolating on the upper surface of the glass a poolof a molten alloy selected from the group consisting of molten alloys oftin, molten alloys of lead and molten alloys of bismuth, and modifyingthe constitution of the upper surface of the glass by passing anelectric current through the glass from said pool of molten alloy intothe molten metal surface.

11. A method according to claim '10, wherein the alloy is an alloy oftin with an element selected from the group consisting of lithium,sodium, potassium, zinc, magnesium, aluminium, silicon, titanium,manganese, chromium, iron, and the rare earth metals.

12. A method according to claim 10, wherein the alloy is an alloy ofbismuth with an element selected from the group consisting of lithium,sodium, zinc, magnesium, aluminium, silicon, titanium, manganese,chromium, iron, cobalt, nickel, copper, silver, gold, antimony, arsenic,indium, the platinum group metals and the rare earth metals.

13. A method according to claim 10, wherein the alloy is an alloy oflead with an element selected from the group consisting of lithium,sodium, zinc, magnesium, aluminium, silicon, titanium, manganese,chromium, iron, cobalt, nickel, copper, sliver, gold, antimony, arsenic,indium, the platinum group metals and the rare earth metals.

14. A method according to claim 3, wherein flat glass in ribbon form isadvanced along a molten metal surface and wherein two pools of moltenmetal are confined on the upper surface of the glass and are separatedby a distance such that the pools are electrically isolated from eachother, and electric currents are passed through the glass between themolten metal surface and both pools.

15. A method according to claim 14, wherein one pool is connected as ananode and the other pool is connected as a cathode with respect to themolten metal surface.

16. A method according to claim 14, wherein the pools are of dissimilarmolten metal, both pools are connected as anodes with respect to themolten metal surface, so that a metal enters the glass surface from thefirst pool, and the surface characteristic so produced is modified bymigration of a second metal into the glass from the second pool.

17. A method according to claim 16, wherein metal ions in a reducedstate enter the glass surface from the first pool.

References Cited FOREIGN PATENTS 620,787 7/1962 Belgium.

S. LEON BASHORE, Primary Examiner JOHN H. HARMAN, Assistant ExaminerU.S. Cl. X.R. 6599; 117-124-

